This application is based on Japanese Patent Application No. 2003-309718 filed on Sep. 2, 2003, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a semiconductor integrated circuit comprising a heat-producing circuit element that produces relatively intense heat and performs a predetermined operation and a plurality of temperature-dependent circuit elements that show a constant relationship in properties with each another while performing a predetermined operation depending on temperature thereof.
2. Description of the Prior Art
For example, in audio equipment, since accuracy in reproduction of sounds deteriorates if an output voltage of a power supply circuit becomes unstable, a switching regulator, a DC/DC controller, or the like is used as the power supply circuit. This type of power supply circuit comprises a power transistor for producing a large output power and a control circuit including a plurality of transistors for controlling the power transistor. A semiconductor integrated circuit arranged in a single chip having the power transistor and the control circuit formed together therein is widely used.
FIG. 5 shows a layout of a power supply circuit formed by a conventional semiconductor integrated circuit. As shown in FIG. 5, the conventional semiconductor integrated circuit has, on a silicon wafer 1, a power transistor 2, a pair of transistors 3 and 4 forming a part of an overheating protection circuit to protect the power transistor 2 from overheating, and a pair of transistors 5 and 6 forming a part of a control circuit to regulate the power transistor 2.
The overheating protection circuit is configured so as to include a comparator for detecting a temperature produced from the power transistor 2. This comparator compares a reference voltage with an input voltage that fluctuates according to the temperature. The transistors 5 and 6 used for the control circuit form a current mirror circuit that performs a control for stabilizing the output voltage of the power transistor 2.
Now, in the semiconductor integrated circuit configured as described above, operating accuracy of the power transistor 2 may deteriorates when heat released therefrom is conducted to the transistors 3, 4, 5, and 6 by heat conduction, because the power transistor 2 produces relatively intense heat in comparison with other circuit elements.
Each of the pair of transistors 3 and 4 that forms the comparator has identical properties with each other. If distances of the transistors 3 and 4 from a heat producing source of the power transistor 2 are different from each other, then temperatures of the transistors 3 and 4 become different from each other. As a result, the properties of the transistors 3 and 4 will be different from each other, causing the aforementioned result.
Therefore, this presents an identical situation in which the transistors 3 and 4 having properties different from each other are used. Consequently, the accuracy of the comparator is decreased and the overheating protection circuit is prevented from serving its original function for protecting the power transistor 2 from overheating and the semiconductor integrated circuit may be destroyed by overheating.
Also, each of the pair of transistors 5 and 6 that forms the current mirror circuit has properties identical with each other. If distances of the transistors 5 and 6 from the heat producing source of the power transistor 2 are different from each other, then temperatures of the transistors 5 and 6 become different from each other. As a result, the properties of the transistors 5 and 6 will be different from each other.
Therefore, this presents an identical condition in which the transistors 5 and 6 having properties different from each other are used. Consequently, the current mirror circuit is prevented from serving its original function so that the accuracy of the control circuit for controlling the power transistor 2 decreases. As a result, the output voltage of the power transistor 2 becomes unstable, leading to, for example, deterioration in the audio reproduction accuracy of the audio equipment.
In a semiconductor integrated circuit incorporating a power transistor that produces a great amount of heat as described above, a comparator that requires high accuracy therein is influenced by the heat in its operation. One of the reasons for this is a distance to the power transistor in the layout of the semiconductor integrated circuit or a thermal conductivity of the material used for the semiconductor integrated circuit. Therefore, in a semiconductor integrated circuit, an offset voltage is generated by a shift in a threshold voltage caused when the uniform operation of the comparator or the like varies due to heat. Because of this reason, the semiconductor integrated circuit should be designed with due attention paid to the layout therein.
In order to solve the aforementioned deficiencies, according to a conventional technology described in the Japanese Patent Registered No. 3400098, each temperature-dependent element is laid by determining its distance and/or direction relative to a heat source element so that two or more temperature-dependent elements maintain a constant relationship in properties even when temperatures of the two or more temperature-dependent elements vary. As a result, the two or more temperature-dependent elements maintain a constant relationship with each another in properties regardless of heat produced by the heat source element.
However, it is possible that the intended accuracy in operation will suffer because the constant relationship in properties among the two or more temperature-dependent elements is not necessarily maintained due to changes of environmental factors. In addition, attempting to solve the deficiencies, by contrast, imposes a restriction on the layout design of the circuit elements.